The new SparseLU module checks whether the matrix in question is already in compressed mode. Unfortunately, when decomposing a MappedSparseMatrix, a compile error will occur as the MappedSparseMatrix does not have a isCompressed() function.
If I'm not mistaken, the MappedSparseMatrix is always wrapping compressed data, so it should be correct to add a isCompressed() function that always return true.

Created attachment 372[details]
Patch that implements what's proposed in the bug report
I just added an isCompressed() member function in MappedSparseMatrix that always returns true. I believe that is consistent with the design and intentions of MappedSparseMatrix.